Hollow twisted and drawn cables and method for making the same

ABSTRACT

Electrical cables of the present invention include cables made from plated filaments which are first twisted together and then drawn through reducing dies (or swaged). The cables exhibit a conductivity comparable to cables having greater diameter and weight. The smaller diameter of the cables of the invention allows them to be used as leads for electronic components in order to achieve reduced parasitic capacitance without increased resistivity or reactance or component package size. The cold working of the cables of the invention provides them with enhanced flexibility and fatigue strength. The combination of materials used in the cables of the invention provides them with resistance to corrosion and the adverse affects of aging as well as enhanced conductivity. Cables formed according to the invention with a hollow tube core can be self-cooling, or easily cooled by flowing coolant through the hollow core.

This application is a continuation of application Ser. No. 08/963,686filed Nov. 4, 1997, now U.S. Pat. No. 6,049,042, which is acontinuation-in-part of application Ser. No. 08/843,405 filed on May 2,1997, now U.S. Pat. No. 5,994,647, the complete disclosures of which arehereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to electrical cables. More particularly, theinvention relates to electrical cables which exhibit low resistance,high fatigue strength, low weight, good flexibility, cool operation,minimized parasitic capacitance, and which are resistant to the adverseaffects of aging and corrosion.

2. State of the Art

My prior application which is referenced above describes the generaltechniques known in the art for making electrical cables from helicallytwisted filaments and proposes methods of twisting and drawing wirecables for enhancing the conductivity, flexibility and tensile strengthof the cables. In addition to low resistance, flexibility and tensilestrength, other characteristics of cables may be important depending onthe application in which the cable is used. For example, the ability ofa cable to remain cool during operation is often an importantconsideration. For cables used outdoors for power transmission,resistance to corrosion and low weight of the cable are importantconsiderations. For cables which are subjected to repeated flexion, goodflexibility as well as high fatigue strength are important. In cableswhich are used as leads for semiconductors and other electroniccomponents, parasitic capacitance is an important consideration.

Among the many factors which affect the resistance of an electricalconductor is its temperature. As the temperature of the conductor rises,so does its resistance. Moreover, as the resistance of a conductor isincreased, current passing through the conductor will further heat theconductor. Known techniques for cooling electrical conductors arecomplex and expensive.

The usual method for preventing or minimizing the effects of corrosionon an electrical conductor is to cover it with insulation. However, inmany applications such as power transmission cables, insulation cansignificantly add to the cost of the cable. Most “high tension” powertransmission cables are not covered with insulation.

In most cables, their weight is dictated by the choice of materials andthe dimensions of the cable. Often, attempts to reduce the weight of thecable results in either increased cost of the materials used tofabricate the cable or an increase in the resistivity of the cable.

Fatigue strength is an important characteristic of electrical conductorswhich are subjected to flexion such as overhead power cables, flexiblepower cords, communications cables, and wires used in hand held orportable equipment. The usual method of increasing fatigue strength isto utilize a stranded conductor rather than a solid conductor. Asexplained in my earlier application, a stranded conductor with the sameconductivity of a solid conductor will have a greater cross sectionaldiameter than the solid conductor. The larger stranded conductor alsorequires more insulation and has increased parasitic inductance. Inaddition, as the overall diameter of the insulated stranded conductorincreases, so does its stiffness. In general, the stiffness ofcylindrical bending beams tends to increase exponentially (to the fourthpower) as the diameter is increased. Also, as the number of strands isincreased, the ratio of surface area to cross sectional area increases.This makes the cable more vulnerable to the damaging effects ofcorrosion which significantly decrease the conductivity of the cable asit ages in service.

Parasitic capacitance is a characteristic of electrical conductors whichis very important in some applications such as leads for semiconductorsand other electrical components. The most common technique forminimizing parasitic capacitance is to make the conductive leads assmall as possible and to separate them as much as possible. However,making the leads smaller (in diameter) increases their resistance; andseparating the leads from each other results in a larger package sizefor the component.

As mentioned above, stranded conductors have increased parasiticinductance as compared to solid conductors of the same diameter. This ismostly the result of the individual strands being helically wound ratherthan being arranged parallel to the axis of the conductor. To the extentthat the individual strands are not in perfect electrical contact witheach other along their entire length, they tend to behave as individualhelical conductors, i.e. as coils. Stranded cables with strands havingcircular cross section always exhibit imperfect electrical contact amongthe strands because they contact each other only along a single line.Cables having preformed strands (e.g. “trapezoidal wire”) also exhibitimperfect electrical contact among the strands because they contact eachother only along portions of their surfaces. An attendant and relatedproblem occurs with customary stranded cables as they age in thepresence of air, moisture and other corrosive agents. The buildup ofsurface corrosion on the strands further reduces the electrical contactamong the strands further increasing parasitic inductance.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an electricalcable which has low electrical resistance as well as a method for makingthe cable.

It is also an object of the invention to provide an electrical cablewhich has a structure which keeps the cable cool.

It is another object of the invention to provide an electrical cablewhich has high fatigue strength, low weight and good flexibility.

It is still another object of the invention to provide an electricalcable which is resistant to the adverse affects of aging and corrosion.

Yet another object of the invention is to provide an electrical cablewhich exhibits reduced parasitic capacitance and inductance.

In accord with these objects which will be discussed in detail below,the electrical cables of the present invention include cables made fromplated filaments which are first twisted together and then drawn throughreducing dies (or swaged), filaments which are twisted together around acore material which melts or otherwise deforms during drawing of thecable through reducing dies (or swaging of the cable), filaments whichare twisted around a tube prior to drawing through reducing dies (orswaging), and cables which are made from combinations of these methods.Presently preferred plating materials include silver and gold. Presentlypreferred core materials include silver and solder. A presentlypreferred core tube is a steel tube.

The cables of the invention exhibit a conductivity comparable to cableshaving greater diameter and weight. The smaller diameter of the cablesof the invention allows them to be used as leads for electroniccomponents in order to achieve reduced parasitic capacitance withoutincreased resistivity or component package size. The cold working of thecables of the invention provides them with enhanced flexibility andfatigue strength. The combination of materials used in the cables of theinvention provides them with resistance to corrosion and the adverseaffects of aging as well as enhanced conductivity. Cables formedaccording to the invention with a hollow tube core can be cooled byflowing a coolant through the hollow core. In addition, the hollow coreprovides improved heat rejection properties because no heat is generatedin the core where current does not flow. The hollow tube core alsoenhances fatigue strength, resists the effects of aging, and lowers theweight of the cable. Cables formed with a silver core, as opposed to ahollow core, are also self-cooling by means of having a lower resistancethermal path from the core of the cable to its surface.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a cable assembly according to afirst embodiment of the invention prior to drawing through reducingdies;

FIG. 2 is a cross sectional view of a finished cable assembly accordingto a first embodiment of the invention;

FIG. 3 is an enlarged view of the portion labelled A in FIGS. 2 and 5;

FIG. 4 is a cross sectional view of a cable assembly according to asecond embodiment of the invention prior to drawing through reducingdies;

FIG. 5 is a cross sectional view of a finished cable assembly accordingto a second embodiment of the invention;

FIG. 6 is a cross sectional view of a cable assembly according to athird embodiment of the invention prior to drawing through reducingdies;

FIG. 7 is a cross sectional view of a finished cable assembly accordingto a third embodiment of the invention;

FIG. 8 is a cross sectional view of a cable assembly according to afourth embodiment of the invention prior to drawing through reducingdies;

FIG. 9 is a cross sectional view of a finished cable assembly accordingto a fourth embodiment of the invention;

FIG. 10 is a cross sectional view of a cable assembly according to afifth embodiment of the invention prior to drawing through reducingdies; and

FIG. 11 is a cross sectional view of a finished cable assembly accordingto a fifth embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 through 3, a first embodiment of a cable 10according to the invention includes three copper (or aluminum) strands12, 14, 16 which are twisted around a central strand 18 which ispreferably made of silver, gold or solder. The bundle of twisted strandsis then drawn, as described in the previously incorporated parentapplication, through at least one reducing die such that the overalldiameter of the twisted assembly is reduced by at least approximately9%, preferably through at least two dies such that the overall diameteris reduced by at least approximately 18%, and more preferably through aplurality of reducing dies such that the overall diameter of the twistedassembly is reduced, preferably by at least 30-40%. Alternatively, thebundle may be swaged to reduce its diameter. The drawing (or swaging)process causes the inner core 18 to flow into the interspaces 12 a, 14a, 16 a between the copper (or aluminum) strands 12, 14, 16 as shownbest in FIGS. 2 and 3. The resultant cable 10 possesses lower resistancethan a cable of the same diameter made of three twisted copper (oraluminum) strands. The central silver core 18 enhances the electricalconnection of the three strands 12, 14, 16 and also provides aself-cooling center of the cable. Since the thermal conductivity ofsilver is greater than that of copper (or aluminum), its electricalconductivity is higher than that of copper (or aluminum) at the sametemperature. The overall resistivity of silver is lower than that ofcopper (or aluminum). Thus, as the cable 10 warms from the center, itsconductivity remains higher than a similar cable without a silver core(or without a core of higher conductivity). If the central core 18 ismade of gold, it will also possess an inherently higher conductivity andthe cable will be less affected by heat than a similar cable without thecore because an improved thermal path exists to conduct heat from thecore to the surface of the conductor. It will also be appreciated thatwhen solder is used as the core, the cable may be more easily solderedto an electrical connection or to another cable.

The above-described cable having a conductive filler material among thestrands will have increased conductivity and reduced parasiticinductance as compared to a stranded cable of similar size with nofiller material. Increased conductivity will result from the intimateelectrical connection established among the strands by the conductivefiller material. This connection will allow current to flow more freelyalong the longitudinal axis of the cable rather than along longer andhigher resistance paths of individual helical strands. In addition, byallowing the current to flow parallel to the longitudinal axis,parasitic inductance is significantly decreased. It will be appreciatedthat when the cable is used to carry an alternating current, the cablewill exhibit reduced inductive reactance for these same reasons.

The methods described above may be applied to a cable having more thanthree strands. For example, as shown in FIGS. 4 and 5, according to asecond embodiment of the invention, four copper (or aluminum) strands12, 14, 16, and 20 are twisted around a central core 18 which ispreferably made of silver, gold or solder. The twisted strands are thendrawn through a plurality of reducing dies as described in thepreviously incorporated parent application such that the overalldiameter of the twisted assembly is reduced, preferably by at leastapproximately 18%, and more preferably by at least 30-40%. The drawing(or swaging) process causes the inner core 18 to flow into theinterspaces 12 a, 14 a, 16 a, 20 a between the copper (or aluminum)strands 12, 14, 16, 20 as shown best in FIGS. 3 and 5. The resultantcable 30 has similar properties as the cable 10 described above. Thoseskilled in the art will appreciate that more than four copper (oraluminum) strands may also be twisted around a central core and drawn orswaged as described above.

Turning now to FIGS. 6 and 7, a cable 50 according to a third embodimentof the invention is made from four copper (or aluminum) strands 12, 14,16, 20 which are each plated with an outer coating 12′, 14′, 16′, 20′ ofgold or silver. The strands are twisted and the twisted strands aredrawn through a plurality of reducing dies as described in thepreviously incorporated parent application such that the overalldiameter of the twisted assembly is reduced, preferably by at leastapproximately 18%, and more preferably by at least 30-40%. The drawing(or swaging) process causes the plating 12′, 14′, 16′, 20′ to flowtogether as shown in FIG. 7. As shown in FIG. 7, a hollow core 28remains at the center of the cable after the twisted strands are drawnor swaged. This may be advantageous to keep the cable cool as discussedabove. However, the cable 50 may be drawn with a central core like thecables 10 and 30 described above to maximize conductive cross sectionalarea.

The cable 50 according to the invention provides several otherinteresting advantages. For example, given that the plating materialcovering the strands is more resistant to corrosion than the copper oraluminum strands, the cable 50 will exhibit superior resistance tocorrosion and the effects of aging. As explained above, this willprevent the cable from developing increased resistance over time andwill forestall increases in parasitic inductance. Thus, plating thestrands with silver, palladium, or gold and drawing or swaging asdescribed above provides increased conductivity as well as asignificantly longer useful life for the cable. It is believed that theadvantages gained in conductivity and longer useful life areeconomically significant relative to the increased cost of plating thestrands. For example, plating or otherwise coating strands with a thin(approx. 0.1 to 10 microns) layer of gold or palladium should preventcorrosion. Plating or otherwise coating strands with a layer of silver(approx. 10 to 500 microns) will result in a cable which, when exposedto common corrosive agents, will develop a conductive silver oxide orconductive silver salt on its surface. While the silver coating does notprevent corrosion, it prevents the adverse effects of corrosion on theelectrical properties of the cable. Coating the strands with layers ofthis thickness and of these materials will not add significantly to thecost of manufacture compared to the value in improved performance forthe cable.

From the foregoing, those skilled in the art will appreciate that thecombined process of plating, twisting, and drawing or swaging providesan unexpected combination of advantages which include: enhancedconductivity, increased useful life, prevention against the effects ofcorrosion, and reduced parasitic inductance. Moreover, it will beappreciated that the combined advantages are greater than what would beexpected from combining the individual effects of plating, twisting, anddrawing.

FIGS. 8 and 9 illustrate a cable 60 according to a fourth embodiment ofthe invention. Four copper (or aluminum) strands 12, 14, 16, 20 aretwisted around a hollow tube 25 which is preferably made of stainlesssteel. The assembled strands and tube are drawn through a plurality ofreducing dies as described in the previously incorporated parentapplication such that the overall diameter of the twisted assembly isreduced, preferably by at least approximately 18%, and more preferablyby at least 30-40%. The resulting cable 60 is provided with a hollowcore formed by the tube 25. According to an alternate embodiment of theinvention, the tube 25 may be filled with a coolant, or a coolant may bepumped through the tube 25.

Turning now to FIGS. 10 and 11, a cable 70, according to a fifthembodiment of the invention, includes three copper (or aluminum) strands12, 14, 16, which may be plated as described above, and four silverstrands 18, 19, 21, 23. The three copper (or aluminum) strands 12, 14,16 are twisted around one of the silver strands 18, and the other threesilver strands 19, 21, 23 are laid in the spaces between the copper (oraluminum) strands and twisted as shown in FIG. 10. The bundle of strandsis drawn through a plurality of reducing dies as described in thepreviously incorporated parent application such that the overalldiameter of the twisted assembly is reduced, preferably by 30-40%. Thedrawing (or swaging) process causes the silver to flow into and fill thespaces between the copper (or aluminum) strands as shown in FIG. 11.

As mentioned above, the fatigue strength of electrical cables is animportant factor for cables subjected to repeated flexion. The presentinvention, as demonstrated in the foregoing exemplary embodiments,provides a higher fatigue strength than conventional stranded cables forthree reasons. First, the reduced overall diameter and more compact formof a cable according to the invention will have a smaller crosssectional area as compared to a stranded cable having similarconductivity. Thus, the cable according to the invention will have asmaller section polar moment which will result in lower stress andstrain on the strands when the cable is bent around a given radius. Thereduction in stress and strain results in a higher fatigue life. Second,by increasing the degree of twist in a stranded cable, the flexibilityof the cable can be increased. Normally this is not recommended sinceincreasing the twist angle adversely affects conductivity and increasesparasitic inductance, especially after aging. With the presentinvention, however, the adverse effects of a high twist angle aremitigated by drawing or swaging. Therefore, the increased flexibility ofa high twist angle can be achieved without the adverse effects. Third,the effects of cold working during drawing or swaging of copper andaluminum conductors causes an increase in yield strength in thesematerials, which also improves the fatigue life of the cable. Fourth,drawing strands of wires through dies produces a highly polishedsurface, as the surface is burnished during the drawing process. Such asurface facilitates the extrusion of an insulative coating over thecable.

While all of the exemplary embodiments given herein refer to ohmicconductors such as copper, aluminum, cold, and silver, it should beunderstood that the advantages of the invention also apply to cableswhere the strands or the plating on the strands or the conductive fillerare made of superconductive material, including low-temperaturesuperconductors such as niobium—titanium, or high-temperaturesuperconductors such as copper-based ceramic formulations known in theart.

There have been described and illustrated herein several embodiments ofelectrical cables and methods of making electrical cables. Whileparticular embodiments of the invention have been described, it is notintended that the invention be limited thereto, as it is intended thatthe invention be as broad in scope as the art will allow and that thespecification be read likewise. It will therefore be appreciated bythose skilled in the art that yet other modifications could be made tothe provided invention without deviating from its spirit and scope as soclaimed.

What is claimed is:
 1. A cable, comprising: a plurality of strandstwisted to form a bundle in which no one of said plurality of strandsforms a center core strand of said bundle, said bundle drawn through atleast one die or swaged to form a cable having a hollow center afterbeing drawn.
 2. A cable according to claim 1, wherein: said plurality ofstrands is exactly four strands.
 3. A cable according to claim 1,wherein: said cable has a diameter reduced by at least 9% relative tosaid bundle.
 4. A cable according to claim 1, wherein: said cable has adiameter reduced by at least 18% relative to said bundle.
 5. A cableaccording to claim 1, wherein: said cable has a diameter reduced by atleast 30% relative to said bundle.
 6. A cable according to claim 1,wherein: at least one of said strands includes a base first material andis plated with a second material different than said first material. 7.A cable according to claim 6, wherein: said second material is softerthan said first material.
 8. A cable according to claim 6, wherein: saidsecond material is more conductive than said first material.
 9. A cableaccording to claim 6, wherein: said first material is one of copper andaluminum.
 10. A cable according to claim 6, wherein: said secondmaterial is one of silver, gold, and palladium.
 11. A cable, comprising:a plurality of strands each having a same non-circular transversecross-sectional shape such that together said plurality of strands forma cable having a substantially circular cross-section and an interiorlongitudinal hollow portion through a longitudinal axis of said cable.12. A cable according to claim 11, wherein: at least one of said strandsincludes a base first material and is plated with a second materialdifferent than said first material.
 13. A cable according to claim 12,wherein: said second material is softer than said first material. 14.36. A cable according to claim 12, wherein: said second material is moreconductive than said first material.
 15. A cable according to claim 12,wherein: said first material is one of copper and aluminum.
 16. A cableaccording to claim 12, wherein: said second material is one of silver,gold, and palladium.
 17. A method of making an electrical cable,comprising: a) twisting a plurality of strands to form a bundle in whichno one strand is a central core strand, wherein at least one of saidstrands includes a base first material and is plated with a secondmaterial different than said first material; and b) compacting saidbundle by drawing through at least one die or swaging to form aconductive cable having a substantially circular transverse crosssection and a longitudinal hollow portion through a longitudinal axis ofsaid cable.
 18. A method according to claim 17, wherein: said secondmaterial is softer than said first material.
 19. A method according toclaim 17, wherein: said second material is more conductive than saidfirst material.
 20. A method according to claim 17, wherein: said firstmaterial is one of copper and aluminum.
 21. A method according to claim17, wherein: said second material is one of silver, gold, palladium. 22.A method according to claim 17, wherein: said cable has a diameterreduced by at least 9% relative to said bundle.
 23. A method accordingto claim 17, wherein: said cable has a diameter reduced by at least 18%relative to said bundle.
 24. A method according to claim 17, wherein:said cable has a diameter reduced by at least 30% relative to saidbundle.